Process for pattern searching and a device for positioning of a mask to a workpiece

ABSTRACT

A process for pattern searching in which position coordinates of patterns on a workpiece with a rough surface can be determined automatically and with high precision, and a device for executing the process are achieved by CCD cameras and the like picking up patterns which are located on a workpiece with a rough surface, and which are formed only by intersecting lines, and by a storage means storing light-dark signals. Furthermore, according to the invention, the image signals are integrated in directions which are each parallel to the lines of the patterns, by which integral signals ΣXn, ΣYn are determined. By integrating the image signals, the images which are formed as a result of surface roughness are averaged. The integration signals clearly yield boundary positions of the patterns, and by differentiating these integral signals, the pattern positions are determined based on their peak positions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for searching patterns for thepurpose of positioning or the like in an exposure for the production ofsemiconductor components, printed boards, liquid crystal displays,printer heads of the inkjet type, multichip modules and the likeprocess, and also for determining the marks in a chip mounter, a bonder,a probing tester and the like and for similar purposes. The inventionfurthermore relates to a device for positioning of a mask to aworkpiece. The invention relates especially to a process for searchingpatterns, a process which is suitable for determining marks on aworkpiece with a rough surface, and a device for positioning of a maskto a workpiece. In particular, to such a device which can be used, notonly for searching patterns on a semiconductor substrate, but also forsearching patterns on a ceramic substrate, a metal and a thin metallicplate, a printed board, a glass substrate which is not polished to ahigh gloss, and the like.

2. Description of Related Art

Production of semiconductor components, printed boards, liquid crystaldisplays, printer heads of the inkjet type, multichip modules and thelike includes an exposure process. In this exposure process, it isimportant in the transfer of the mask pattern onto the workpiece thatanother pattern to be subsequently transferred is exactly positionedrelationship to a pattern which has been formed beforehand. Theabove-described positioning is conventionally done such that thealignment marks which are recorded on the mask and the workpiece come torest on top of one another.

FIG. 7 schematically shows the arrangement of a device for determiningthe alignment marks on a workpiece. In the drawings reference letters AUindicate an alignment unit which consists of a half mirror M, lenses L1,L2 and CCD camera 11. Furthermore, reference number 12 identifies amonitor and numeral 13 an arithmetic-logic unit. The workpiece on whichalignment marks AM are projected or recorded is designated W.

In the following, a pattern search of alignment marks AM on workpiece Wis described using FIG. 7.

First, alignment marks AM are recorded in arithmetic-logic unit 13. Forexample, cross-shaped marks as in FIG. 8(a) are used as alignment marksAM. In arithmetic-logic unit 13, patterns of alignment marks AM arerecorded, in which monitor pixels are regarded as unity, as isillustrated in FIG. 8(b). In the figure, the number of pixels is set toa 5×5 matrix to facilitate the explanation.

Next, as is shown in FIG. 7, illumination light is transmitted via thehalf mirror M of alignment units AU onto alignment marks AM on workpieceW. Alignment marks AM are picked up by CCD cameras 11, one of which isdepicted in FIG. 7. Furthermore, images of alignment marks AM which havebeen projected onto monitor 12 are input into arithmetic-logic unit 13and converted into coordinate data, the pixels of the monitor beingconsidered unity. In arithmetic-logic unit 13, the above describedrecorded patterns are compared to the images of the picked-up alignmentmarks.

The case in which the images of the picked-up alignment marks (searchedpatterns) correspond to searched pattern A in FIG. 8(c) is recognized asscore 60 because agreement with the recorded pattern is around 60%.

The case in which the images of the picked-up alignment marks (searchedpatterns) correspond to searched pattern B in FIG. 8(d) is recognized asscore 80 because agreement with the recorded pattern is around 80%.

The case in which the images of the picked-up alignment marks (searchedpatterns) correspond to searched pattern C in FIG. 8(e) is recognized asscore 100 because agreement with the recorded pattern is around 100%.

When patterns with a score of roughly 100 are recognized, these patternsare recognized as searched patterns, as was described above.

In the production of semiconductor components and the like, for purposesof cost reduction and the like there are cases in which an etched waferwhich has not undergone high gloss polishing is used, when a pattern isformed on the wafer with a line width which is not extremely fine(surface roughness is different depending on the users).

FIG. 9 schematically shows one example of an image of the abovedescribed wafer which has not undergone high gloss polishing. As isshown in the drawing, in the case of a wafer which has not undergonehigh gloss polishing, images with prominent contrasts occur as a resultof surface roughness such that they and the image of the alignment markcome to rest on top of one another, or that they are located on theperiphery hereof.

If, when the alignment marks which are located on a wafer of this typeare recognized, the above described pattern search method is undertaken,the above described images as a result of surface roughness are sent assignals with prominent contrasts from the CCDs camera and the imagesignals of the searched patterns of the images due to surface roughnessare added to the coordinate data of the searched patterns. As a result,the score decreases, and a pattern search can no longer be done.

This also applies in workpieces besides the above described wafer whichhas not been subjected to high gloss polishing. In the above mentionedceramic substrate, metal and the metallic thin plate and the like, thereare a host of cases in which a search of the alignment marks by theabove described pattern search process is difficult to perform.

SUMMARY OF THE INVENTION

The invention was made to eliminate the above described disadvantages ofthe prior art. Thus, a primary object of the invention is to devise aprocess by which patterns on a workpiece with a rough surface can besearched automatically and with high precision, and a device forexecuting the process.

The above described object is achieved in accordance with embodiments ofthe invention as follows:

(1) To determine the position coordinates of patterns on a workpiecewith a rough surface, images of the patterns which are formed only byintersecting parts of lines are stored as light-dark signals. The abovedescribed light-dark signals are integrated into directions which areeach parallel to the lines of the above described patterns, by whichintegral signals ΣX_(n), ΣY_(n) are determined, which each correspond tothe above described lines, as is illustrated in FIG. 2. In this way, theimages which have been formed as a result of surface roughness areaveraged. The above described integral signals become signals whichclearly indicate the edge transitions of the patterns, as is illustratedin FIGS. 2(a) and 2(d).

Next, the above described integral values are each differentiated, bywhich differential signals are obtained which are shown in FIG. 2(b) and2(e). The pattern positions are determined using the positions of theirpeak values.

(2) The process described above in (1) for pattern searching is used fora device for positioning of a mask to a workpiece. In doing so, theposition coordinates of alignment marks of a workpiece with a roughsurface or of alignment marks of a mask which are imaged on a workpiecewith a rough surface are determined, and thus, positioning of a maskrelative to a workpiece is performed.

In the invention, automatic determination of the pattern positions onthe workpiece with a rough surface is enabled with high precision by themeasure by which the patterns are searched in the manner described in(1) above.

In the following, the invention is further described in specific termsusing the embodiment shown in the drawing. In the embodiment describedbelow a case is described in which the invention is used for positioningof a mask relative to a workpiece in a projection exposure device. But,the invention is not limited to the embodiment described below. Theinvention can also be used for contact exposure, proximity printing, orpattern searching in a chip mounter, bonder, probing tester and thelike, as is described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic depiction of an embodiment of a deviceaccording to the invention for positioning of a mask relative to aworkpiece;

FIG. 2 shows a schematic of the process according to the invention forsearching patterns;

FIGS. 3(a) & 3(b) are examples of the alignment marks used according tothe invention;

FIG. 4 shows a schematic of a process for determining the positions ofthe alignment marks;

FIG. 5 shows a schematic of a process for determining an average length;

FIGS. 6(a) & 6(b) show examples of alignment marks which are consists ofsegments which do not orthogonally intersect;

FIG. 7 shows a schematic of the arrangement of a prior art device fordetermining the position of alignment marks;

FIG. 8 shows a schematic of a conventional pattern search process; and

FIG. 9 is a photograph of an example of the image of a wafer which hasnot undergone high gloss polishing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a schematic illustration of an embodiment of a projectionexposure device according to the invention having a light irradiationdevice LH for emitting exposure/nonexposure light, which is passedthrough a filter F for passage of nonexposure light, and a mask M, whichis provided with a mask pattern (not shown) and mask alignment marks MAMfor purposes of positioning (in the following called only mask marksMAM). Furthermore, a mask carrier MS is provided which can be moved inthe X-Y directions (two axes which orthogonally intersect one another ona plane parallel to the mask surface), in the Z-direction (one axiswhich orthogonally intersects the X-Y plane), and in the θ direction(one revolution around the Z-axis), and on which above mask M is placedand attached by means of a vacuum chuck or the like, as is shown in thedrawing.

Furthermore, a projection lens L3 and a workpiece W with a surface whichis rough are shown. On the workpiece W, workpiece alignment marks WAM(hereinafter called only workpiece marks WAM) are recorded. Furthermore,reference letters WS labels a workpiece carrier on which above describedworkpiece W is placed and attached by means of a vacuum chuck or thelike.

An alignment unit AU comprises a half-mirror HM, lenses L1, L2, and CCDcamera 1, as was described above. Here, in the case of a projectionexposure device, the optical path deviates from the projection images ofmask marks MAM in above described half mirrors HM, when half mirrors HMhave a certain thickness. In this way, the imaging positions of maskmarks MAM on workpiece W change and a correction must be obtainedaccording to this change. Half mirrors M are formed conventionally of aresin film, for example, a pellicle with a thickness of a few microns orthe like. In this way, the above described effect of thickness isreduced to an amount at which negligible problems occur with respect touse.

Although not shown in the drawing, usually there are several alignmentmarks (at least at two locations) on mask M and workpiece W, and analignment unit AU is assigned in correspondence with the number ofalignment marks.

Furthermore, reference number 2 identifies a control device andreference number 3 indicates a monitor. Image signals which have beenpicked up by the CCD cameras 1 of above described alignment units AU aresent to the control device 2 which, by the above described method,determines mask marks MAM imaged on workpiece W and workpiece marks WAMon workpiece W, moves mask carrier MS or workpiece carrier WS andexecutes automatic positioning of mask M to workpiece W.

In the following, automatic positioning of mask M relative to workpieceW is described using FIG. 1. The technology of automatic positioning ofmask M to workpiece W in the above described projection exposure deviceis known. For this reason, different processes have been conventionallysuggested. In this embodiment, it is described by way of arepresentative example selected therefrom:

(1) Nonexposure light is emitted onto mask M from light irradiationdevice LH via filter F. Mask marks MAM are projected via the projectionlens L3 onto the workpiece W.

(2) By means of CCD cameras 1 of alignment units AU, images of the maskmarks MAM imaged onto the workpiece W are picked up. Their image signals(light-dark signals) are sent to control device 2 which determines theposition coordinates of the mask marks MAM using the method describedbelow and stores its values. In above described steps (1) and (2) maskmarks MAM can also be imaged on a projection surface without placing ofa workpiece, and the images of the mask marks MAM which were imaged onthe projection surface can be picked up. In this case, exposure lightcan be emitted from the light irradiation device LH, by which imagingerrors or aberrations of the projection lens which are caused by thenonexposure light and exposure light need no longer be considered.

(3) Next, irradiation with nonexposure light is stopped. From anirradiation device of illumination light (not shown), illumination lightor the like is emitted onto workpiece marks WAM on workpiece W(nonexposure light which is emitted from the light irradiation devicecan also be used). The CCD cameras 1 of alignment units AU pick up theimages of workpiece marks WAM on workpiece W and their image signals(light-dark signals) are sent to control device 2 which determines theposition coordinates of workpiece marks MAM by the method describedbelow, and their values are stored.

(4) If the position coordinates of mask marks MAM and workpiece marksWAM have been determined, control device 2 moves mask carrier MS or theworkpiece carrier WS such that the position coordinates of mask marksMAM and the position coordinates of workpiece marks WAM come to rest ontop of one another, and thus automatic positioning of mask M relative toworkpiece W is achieved. After completing positioning of mask M relativeto the workpiece W in the above described manner, alignment units AU areremoved to the extent necessary. Then, exposure light is emitted fromlight irradiation device LH, the mask pattern on mask M is projectedonto the workpiece W, and the workpiece is exposed.

If in the determination of the position coordinates of the mask marksMAM and workpiece marks WAM in above described steps (2) and (3), thesurface of workpiece W is rough, images with prominent contrasts occurdue to surface roughness, as was described above. Therefore, usingconventional methods, the position coordinates of mask marks MAM imagedonto the workpiece W or the position coordinates of the alignment marksWAM recorded on the workpiece W cannot be determined.

Furthermore, in the case of determining the position coordinates of maskmarks MAM without projecting them onto workpiece the W, it is possibleto determine mask marks MAM by the above described conventional processbecause no influence is exerted by the surface roughness of theworkpiece, as was described above. However, due to the surface roughnessof the workpiece, it becomes difficult to determine the positioncoordinates of workpiece marks WAM, because workpiece marks WAM arerecorded on workpiece W. Also, in the case in which mask marks MAM areprojected onto workpiece W, determination of workpiece marks WAM isespecially difficult due to their low contrast, although the images ofmask marks MAM have relatively prominent contrasts and can be easilydetermined.

In this embodiment, alignment marks as in FIG. 3 are used which areformed by lines in a cross-shape, at right angles or in similar ways. Inthe following, the pattern searching for workpiece marks WAM (the maskmarks MAM as necessary) is described. Furthermore, in the following, theworkpiece marks WAM and mask marks MAM are called patterns.

FIG. 2 is a schematic of the process for searching the patterns by meansof the above described control device 2. The drawing shows a cruciformshaped pattern image which is picked up by CCD camera 1 in FIG. 1 and isdisplayed on monitor 3. On monitor 3, an images occur, as is shown inthe drawing, as a result of the surface roughness of the workpiece, suchthat they and the pattern image come to rest on top of one another, orsuch that are located on its periphery as black and milky clouded images(see, FIG. 9). In the FIG. 2 drawing, to facilitate explanation, thecloudy image is shown schematically on monitor 3 as black and hollowcircles. In practice, many more black and milky clouded images occur onmonitor 3, as was described above. Furthermore, in this embodiment, itis assumed that the pattern image is brighter than the image on itsperiphery.

When the image shown in FIG. 2 is picked up, the control device 2integrates the data of the respective pixel along the lines which areparallel to the lines which form the above described pattern. Forexample, in the case in which the pattern is formed by orthogonallyintersecting lines, as is shown in the drawing, for points x1, x2, x3, .. . , xn of the x-axis, which is parallel to the above described line 5,the data of the respective pixel are integrated in the Y-axis direction,by which (x1, Σym), (x2, Σym, . . . (xn, Σym) [m=1 . . . n] isdetermined. Furthermore, at each of points y1, y2, y3 . . . , yn of theY-axis, which is parallel to the above described line, the data of therespective pixel are integrated in the X-axis direction, by which (y2,Σxm), . . . (yn, Σxm) [m=1 . . . n] is determined.

Signals ΣX_(n), ΣY_(n) in FIG. 2(a) and (d) show the above describedintegral values. The black clouded image parts and the milky cloudedimage parts on the workpiece are averaged by integration. As isillustrated in FIG. 2(c), for example, milky cloudiness A, blackcloudiness B, C and the pattern image occur as image signals. Byintegrating them, milky cloudiness A and black cloudiness, C areaveraged, which diminishes the effect on the integral values.

Furthermore, also in the case in which black cloudiness B and thepattern image come to rest on top of one another, as is shown in thedrawing, the effect thereof on the integral value in the Y-axisdirection at point xa hardly occurs, by which the position of thepattern image can be reliably determined.

Therefore, by differentiating the signals ΣX_(n), ΣY_(n) in (a) and (d),signals are obtained which have steep peak values at the edge transitionof the pattern, as is illustrated in FIGS. 2(b) and 2(e). In this way,determination of the edge positions of the pattern is enabled with highprecision.

As was described above, control device 2 first receives integral signalΣX_(n), differentiates this signal, determines and stores peak position(xa, xb) of the differential signal. Next, it obtains the integralsignal ΣY_(n), differentiates this signal, determines and stores peakposition (ya, yb) of the differential signal. Then, middle position (Xc,Yc) is determined by Xc=(xa+xb)/2, Yc - (ya+yb)/2 and the middleposition (Xc, Yc) is fixed as the position of the pattern, as isillustrated in FIG. 4.

For the above described lengths of segments x1-xn, y1-yn to beintegrated (these lengths are called average lengths), roughly, lengthsare necessary with which the black cloudiness and the milky cloudinesswhich can be observed as a result of surface roughness of the workpiececan be cancelled by averaging. These lengths can be determined asfollows:

If, for example, in FIG. 2, image signals along the line in the X-axisdirection or Y-axis direction are observed and the data in FIG. 5obtained, Δ1 means a contrast difference of the pattern with respect tothe reference level and Δ2 is a contrast difference of the blackcloudiness and the milky cloudiness which can be observed as a result ofthe surface roughness of the workpiece with respect to the referencelevel.

Furthermore, the average lengths L are fixed by values d and k, where dis the grain size d of the black and milky cloudiness which can beobserved as a result of the surface roughness of the workpiece and k isa coefficient corresponding to the surface roughness (the greater thesurface roughness, the larger that the coefficient k becomes). Thelarger Δ1, the smaller average lengths L which can be achieved. Thelarger Δ2 and grain size d, the larger average lengths L must be made.This means that average lengths L are selected such that, with respectto value α of the following formula, L>α is obtained:

    α=|Δ2/Δ1|d·k

The amount of surface roughness of the workpiece differs depending onthe types of workpieces, users and the like. The values of the grainsize of the black and milky cloudiness d, the values of Δ1, Δ2 and k arealso different accordingly. Thus, it is desirable that the averagelengths L be selected according to the amount of surface roughness ofthe workpiece.

If, therefore, it is provided, for example, that the magnificationfactor of the alignment units AU can be changed, and if themagnification factor is set according to the amount of surface roughnessof the workpiece, determination of the pattern positions with highprecision for different workpieces is enabled.

In this case, the size of workpiece mark WAM is fixed to be slightlylarger than grain size d of the black and milky cloudiness whichcorresponds to the amount of surface roughness of workpiece W.

In the above described embodiment, it is described that the pattern,with regard to the reference level, is in the positive range (+) (thepattern is lighter). However, the pattern can also be in the negativerange (-) with regard to the reference level (the pattern is darker).

In the above described embodiment, an example was shown in which apattern is used which is formed by orthogonally intersecting lines. Thelines which form the pattern, however, need not orthogonally intersectone another. As is illustrated in FIG. 6, in the case of using a patternwhich is formed by straight lines which do not orthogonally intersect,it is necessary that the above described integral directions areparallel to the lines which form the pattern.

As was described above, the invention is not limited to the projectionexposure device. Alignment units AU are inserted, for example, in thecase of a contact exposure device and a proximity printing devicewithout using projection lens L3 between the light irradiation device LHand mask M, and the workpiece marks WAM are determined through mask MAM.In doing so, mask marks MAM are not projected onto workpiece W and aredetermined directly. Therefore, they can be determined using theconventional process when the effect of the surface roughness ofworkpiece W is not a factor. Determination of the position coordinatesof workpiece marks WAM becomes difficult, however, as a result of thesurface roughness of workpiece W, for which the invention is used.

Action of the Invention

As was described above, automatic determination of the pattern positionon a workpiece with surface roughness is enabled with high precision bythe measure according to the invention by which images of the patternswhich are formed only by parts of straight lines with directions whichintersect are stored as light-dark signals, by which image signals areintegrated in directions which are each parallel to the lines of theabove described patterns, by which integration signals are determinedwhich each correspond to the number of the above described lines, and bywhich the positions of the patterns are determined based on the peakpositions by differentiating the respective integral value.

Therefore, it becomes possible to search patterns on differentworkpieces, such an as an etched wafer, a ceramic substrate, a metal, athin metallic plate, a printed board and the like without manualassistance; this can contribute to automation of searching of thesepatterns.

It is to be understood that although a preferred embodiment of theinvention has been described, various other embodiments and variationsmay occur to those skilled in the art. Any such other embodiments andvariations which fall within the scope and spirit of the presentinvention are intended to be covered by the following claims.

What we claim is:
 1. Process for pattern searching, in which theposition coordinates of patterns formed of straight lines are searchedon a workpiece with a surface roughness, comprising the steps of:storingimages of said patterns, which images are formed only by parts ofstraight lines, as light-dark signals; determining segment lengths oflines to be integrated based on a contrast of said pattern with respectto a reference level and on said surface roughness of said workpiece;integrating the light-dark signals along lines which extend indirections which are each parallel to a respective one of the lines ofthe patterns and which are of the segment lengths determined in saiddetermining step, by which integration signals are determined whichcorrespond to said lines; producing differential signals which representa mathematical differentiation of the integration signals anddetermining actual peak positions of the differential signals; anddetermining pattern positions on the basis of the peak positions whichwere determined; wherein the step of determining segment lengths oflines to be integrated is performed in accordance with the relationship:

    L>|Δ2/Δ1|d·k

where L is the segment length, Δ1 is a contrast difference of thepattern with respect to the reference level and Δ2 is a contrastdifference of black cloudiness and milky cloudiness observed as a resultof the surface roughness of the workpiece with respect to the referencelevel, d is a grain size of said black and milky cloudiness, and k is acoefficient corresponding to said surface roughness.
 2. Processaccording to claim 1, wherein the pattern of which an image formed ofparts of lines is stored is comprised of lines which intersectorthogonally.
 3. Device for positioning of a mask relative to aworkpiece comprising:a mask which is provided with a mask pattern andmask alignment marks formed of a pattern of straight lines; a maskcarrier on which the mask is located; a workpiece with a surfaceroughness on which workpiece alignment marks formed of a pattern ofstraight lines are provided; a workpiece carrier on which the workpieceis located; a light irradiation device for irradiating the workpiecewith light via said mask; alignment units for picking up images of themask alignment marks and the images of alignment marks located on theworkpiece; and a control device which receives image signals from thealignment units and has means for determining position coordinates ofthe alignment marks of the mask and of the workpiece, for driving themask carrier and the workpiece carrier, and for executing positioning ofthe mask relative to the workpiece; wherein the control device has ameans for storing the image signals received from alignment units aslight-dark signals; wherein the means for determining comprises meansfor determining segment lengths of lines to be integrated based on acontrast of said pattern with respect to a reference level and on saidsurface roughness of the workpiece, means for integrating the light-darksignals stored by the storage means along lines which extend indirections which each are parallel to a respective one the lines of thealignment marks, using data along segments of said lines of the segmentlengths determined, to obtain integral values which correspond to eachof the lines of the pattern, and means for producing differentialsignals which represent a mathematical differentiation of the integralvalues and determining actual peak positions thereof, and means fordetermining the positions of the alignment marks on the basis of saidpeak positions; and wherein said means for determining segment lengthsof lines to be integrated comprises means for determining the segmentlengths in accordance the relationship:

    L>|Δ2/Δ1|d·k

where L is the segment length, Δ1 is a contrast difference of thepattern with respect to the reference level and Δ2 is a contrastdifference of black cloudiness and milky cloudiness observed as a resultof the surface roughness of the workpiece with respect to the referencelevel, d is a grain size of said black and milky cloudiness, and k is acoefficient corresponding to said surface roughness.
 4. Device accordingto claim 3, wherein the alignment marks of which the images are formedare comprised of lines which intersect orthogonally.